1. Technical Field of the Invention
The present invention is in the technical field of insertion devices for high energy accelerator based advanced X-ray sources. In order to generate a coherent beam of X-rays from an electron beam, the beam must be periodically deflected in a plane transverse to its axial motion, usually employing a specialized magnet called an undulator. This invention is a new design for an undulator based on an electron drive beam powering a slow-wave microwave waveguide. The electromagnetic fields induced in the waveguide by the drive beam are used to introduce a periodic deflection of a second electron beam propagating through the waveguide in the direction opposite to the drive beam. The second beam then emits high power short wavelength coherent electromagnetic radiation.
It is important to emphasize that the undulator waveguide may be any type of slow-wave structure with an appropriately chosen resonant frequency. Thus the undulator may be based on metallic (normal or superconducting) structures similar to the kind used in conventional particle accelerators, including iris-loaded and corrugated wall structures. Another class of possible undulators for this application is the dielectric loaded structure, exploiting the well-developed technology of the dielectric wakefield accelerator (DWA) for generating the deflecting field using a drive beam. Extensions to the DWA concept can be used to enhance the performance of the BDU such as the addition of dielectric field concentrators to the undulator waveguide. Furthermore, through the inclusion of nonlinear dielectrics or adjustable geometric parameters to the BDU structure, the structure wavelength and hence the wavelength of the X-ray output can be varied.
The principal application of this technology is the generation of X-ray beams for basic materials research, medical therapy and diagnostic imaging.
2. Description of the Background Art
The Beam Driven Undulator (BDU) described in this present invention represents significant improvements over existing designs and concepts for high power short wavelength undulators.
Current undulator technology typically uses a static magnetic field generated by a periodic array of permanent magnets or electromagnets to introduce a periodic deflection in the path of an electron beam that causes it to emit coherent radiation. The undulator wavelength λu is related to the wavelength λ of the emitted radiation by λ˜λu/2γ2, where γ=Ebeam/mc2. Construction of a workable magnetic undulator becomes increasingly difficult as λu is reduced below the 1 cm level required for a coherent X-ray source.
The limitations on the minimum λu in a static field based undulator can be overcome by using a high frequency electromagnetic wave to create the periodic deflecting field. For example, a cavity resonator supporting a hybrid (deflecting) mode driven by a high power rf source could be used as an undulator; a corrugated metallic waveguide for this purpose has been demonstrated in the laboratory. This approach is not in current use because of the Gigawatt-level external microwave source required for its operation. The BDU presented here effectively develops a short undulator period without static deflection magnets and without an external high power microwave source by using a second counter propagating low energy electron beam to generate the requisite deflecting fields.
Further, the nature of existing undulator schemes does not permit changing the frequency of the emitted X-ray beam over a wide range of values.
Further still, in this invention a large beam aperture can be used, contrary to magnetic undulators, allowing greater beam transmission and thus a higher intensity X-ray beam to be produced.
Further still, this invention has the capability of producing both circular and plane polarized X-ray beams.
Further still, this invention allows fast dynamical control of the X-ray beam characteristics, specifically the polarization and wavelength.
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
The present invention is a technique for producing coherent X-ray beams from a high energy electron beam passing through a nonmagnetic undulator device consisting of a slow-wave structure. The periodic deflection force required to generate the X-rays from the primary beam is generated by the electromagnetic wave produced in the undulator structure by a second, low energy electron beam propagating in the opposite direction to the primary beam.